This invention relates generally to concrete building panels, and more particularly to such building panels comprising a laminated structure wherein an inner layer of concrete and an outer layer of concrete have a layer of insulating material sandwiched therebetween. With such panels it is necessary to provide some form of reinforcing means or connectors extending between the layers of concrete through the insulation to prevent delamination failure of the panels.
In the prior art, various means have been used. For example, with early panels of the insulated type, expanded wire mesh was installed between joints in the insulation and extending between the layers of concrete. This type of construction had the disadvantage of requiring a space between the joints of insulation and further the insulation joints needed to be in a special pattern to suit the requirements of the shear connectors. Another prior art method comprised the securement, as by welding or the like, of a piece of reinforcing rod, such as 1/4 inch diameter steel rod, to the reinforcing strands or steel in the bottom or first cast layer of concrete, with a part of the reinforcing rod projecting upwardly. The layer of insulation was then pushed downwardly over the upwardly projecting piece of steel and the steel was thereafter bent over the insulation to anchor the insulation and steel in place. The second layer of concrete was thereafter cast over the layer of insulation. While this arrangement was structurally sound, excessive labor was required to install the upwardly projecting lengths of steel.
Later, with the introduction of prestressed concrete insulated panels, the type of shear connectors used had to be changed, since there is no reinforcing steel in the bottom layer of concrete to which the shear connectors could be attached. However, in such prestressed panels, there generally always were prestressed steel strands present in the top layer of concrete and a U-shaped shear connector was developed which was pushed downwardly through the layer of insulation over the reinforcing strands. The strands thereby regulated the depth to which the connectors were inserted. However, such a construction required that the layer of insulation be installed over the bottom layer of concrete and then the reinforcing strands placed and stressed. This procedure frequently resulted in too much time passing between the casting of the bottom layer of concrete and the placement of the shear connectors, with the result that the concrete in the bottom layer would be partly set up before the shear connectors were installed and the bonding between the concrete in the bottom layer and the shear connectors was thus not sufficient. Consequently, these panels sometimes experienced a delamination failure. The situation was rendered more acute on production lines wherein a long casting bed was used for a long line production or where relatively complicated panels with time consuming steps were utilized. One attempt made to solve the above problem was to make the shear connectors of such a length that when they were pushed through the layer of insulation and the bottom layer of concrete, they would extend into contact with the form for the bottom layer of concrete. However, this structure resulted in a thin layer of concrete over the shear connectors in the bottom layer, which tended to spall from the panel during handling.
The present invention, on the other hand, provides a simple and economical shear connector which may be quickly and easily placed through the layer of insulation and bottom layer of concrete and the shear connector has a projection thereon defining a depth gauge to automatically indicate when the shear connector has been installed to the proper depth. The shear connector may thus be positioned immediately after casting the bottom layer of concrete and prior to the time the reinforcing strands are installed and stressed.